Two-Step Acceleration Model of Cosmic Rays at Middle-Aged SNR

TL;DR

This paper proposes a two-step acceleration model for cosmic rays in middle-aged supernova remnants, explaining observed gamma-ray spectra through shock interactions with interstellar clouds.

Contribution

It introduces a novel two-step acceleration mechanism involving reflected shocks from cloud interactions, supported by 3D magnetohydrodynamics simulations.

Findings

Reflected shocks have Mach numbers around 2, depending on density contrast.

The model reproduces the observed gamma-ray spectral features.

Predicts high-energy spectral index between 3 and 4.

Abstract

Recent gamma-ray observations of middle-aged supernova remnants revealed a mysterious broken power-law spectrum. Using three-dimensional magnetohydrodynamics simulations, we show that the interaction between a supernova blast wave and interstellar clouds formed by thermal instability generates multiple reflected shocks. The typical Mach numbers of the reflected shocks are shown to be M ~ 2 depending on the density contrast between the diffuse intercloud gas and clouds. These secondary shocks can further energize cosmic-ray particles originally accelerated at the blast-wave shock. This "two-step" acceleration scenario reproduces the observed gamma-ray spectrum and predicts the high-energy spectral index ranging approximately from 3 to 4.